Blender bones are confusing. How do they work?
When you are in Pose mode the rotation centre is the head of the bone:



What you show is the upper leg bone parented to the lower spine bone. You're talking about constraints but it's not cleat what you're talking about, in Blender, Constraints are tools used to control objects or bones, like for example Inverse Kinematics constraint, but I don't think there's any constraint between your upper leg and lower spine. I think you mean that these 2 bones are parented, upper leg is the child of lower spine. In that case there's a doted line between the two. This line appears between the parent's tail and the child's head. It won't cause any problem of rotation or please be more specific.



There are 2 kinds of parentage. The first case is the previous one: When you parent a bone to another one but you want the child to stay away you need to choose in Keep Offset mode: select the bone you want to make child, then the parent, then ctrlP > Keep Offset. If you want the child to stick to its parent, choose the Connected bone. If you extrude a bone from another one, it will be a connected child by default. You can separate it definitely with altP > Clear Parent, or keep it the child with altP > Disconnect Bone.



You'll find a lot of tutorials about bones and rigging, explaining everything here would be a bit tedious, I think BlenderStackExchange is more for punctual problems.